Method for producing alkyl nitrites and alkyl dinitrites

ABSTRACT

In a continuous process for the preparation of alkyl nitrites and dinitrites, an alkanol or dialkanol is first mixed with an aqueous mineral acid, the reaction mixture obtained is then reacted with an inorganic nitrite and the product obtained can then be isolated immediately.

The present invention relates to a continuous process for thepreparation of alkyl nitrites and dinitrites by reaction of alkanols ordialkanols with nitrite ions in the presence of an aqueous mineral acid.

Alkyl nitrites and dinitrites are important nitrosation reagents inpreparative chemistry. They are used, for example, to prepare diazoniumcompounds from primary amines. Particularly aromatic diazonium compoundsare important intermediates, since the diazonium group can be convertedinto a multitude of substituents such as pseudohalogen, halogen(Sandmeyer reaction, Schiemann reaction), hydroxyl, mercapto orhydrogen. Diazonium compounds are further used as starting materials inphenanthrene ring closures (Pschorr reaction), for the preparation ofaryl hydrazones (Japp-Klingemann reaction), of aryl hydrazines or of azocompounds.

DE 21 444 20 describes a batchwise process for the preparation ofglycols and glycol derivatives, in which glycols or glycol derivativesare stirred with sodium nitrite and ice-water and hydrochloric acid isthen added, with the temperature being held below 10° C. by means ofexternal cooling.

Organic Syntheses Coll. vol. 2, 108 (1943) describes the preparation ofn-butyl nitrite, in which a mixture of water, iso-propanol and sulfuricacid is added to aqueous sodium nitrite. A disadvantage of this processis the formation of a three-phase mixture with solid sodium sulfate inthe third phase.

U.S. Pat. No. 4,980,496 describes a batchwise process for thepreparation of alkyl nitrites, where an aqueous solution of aC₁-C₅-alkanol is mixed with an alkali metal nitrite, the mixture iscooled to about −10 to 10° C. and then a hydrohalic acid is added whilethe temperature is kept constant. The hydrohalic acid is generally usedin excess, in particular of about 4 mol % based on the alcohol, in orderto achieve a complete conversion.

The process of U.S. Pat. No. 4,980,496 is problematic in variousrespects. Firstly, the reaction is exothermic, so that it is difficultto hold the reaction temperature in the range from −10 to 10° C.Secondly, alkyl nitrites in a strong acidic medium tend to decompose andform by-products, so that the acid excess necessary for the reactionmust be neutralized, or the product must immediately be separated fromthe aqueous phase and stored under suitable drying conditions.

Investigations carried out by the assignee have in turn shown that thereduction of the acid quantity in the batchwise process leads toincreased formation of by-products such as acetals and ketals and to areduction in the yield.

It is an object of the present invention to provide a process for thepreparation of alkyl nitrites and dinitrites whereby these compounds canbe prepared in a simple way and with increased purity and also goodstorage stability. Ideally, the process shall have no special equipmentsand preparative requirements in order that costs may be minimised.

We have found that this object is achieved when an alkanol or dialkanolis mixed with the aqueous solution of a mineral acid using on averagenot more than 1.01 mol of acid equivalents per mole of hydroxyl group ofthe alkanol or dialkanol is used and an aqueous solution of an inorganicnitrite is continuously added to this mixture.

Therefore, the present invention provides a process for continuouspreparation of alkyl nitrites and alkyl dinitrites by reaction of analkanol or a dialkanol with an inorganic nitrite in the presence of atleast one mineral acid that does not oxidize nitrite, wherein

-   -   (i) the alkanol or dialkanol is mixed with an aqueous solution        of the mineral acid using on average not more than 1.01 mol of        acid equivalents per mole of hydroxyl group in the alkanol or        dialkanol is used,    -   (ii) an aqueous solution of the inorganic nitrite is added        continuously to the aqueous mixture obtained in (i) in a        reaction zone, and    -   (iii) the organic phase is optionally isolated.        Examples of aqueous mineral acids that do not oxidize nitrite        include dilute sulfuric acid, phosphoric acid and hydrohalic        acids, e.g. hydrogen chloride, hydrogen bromide or hydrogen        iodide. In a preferred embodiment, hydrogen chloride in water        (hydrochloric acid) is used. The concentration of hydrogen        chloride in water can be varied within a wide range. In general        it is in the range from 10 to 40% by weight, preferably from 15        to 35% by weight. Technical grade hydrochloric acid is        customarily used.

Suitable alkanols and dialkanols are those that have at least somesolubility in water. These include C₂ to C₆ alkanols, such as ethanol,n-propanol, isopropanol, n-butanol, sec-butanol, isobutanol,tert-butanol, n-amyl alcohol, isoamyl alcohol (3-methyl-1-butanol),2-methyl-1-butanol, n-hexanol, 2-hexanol and 2-methyl-1-pentanol.

Suitable dialkanols are selected from the group consisting of C₂ to C₆dialkanols such as 2,2-dimethyl-1,3-propanediol, 1,4-butanediol,1,5-pentanediol, especially 2,2-dimethyl -1,3-propanediol.

The present invention is particularly suitable for the reaction ofn-butanol, tert-butanol and isoamyl alcohol.

The alkanol or dialkanol is generally used in an undiluted form. Asolvent may also be used, and water is preferable. In principle, it isalso possible to use any solvents that are miscible with water can beused, that are inert toward acidic nitrite and that do not restrict theuse of the reaction products, e.g. cyclic ethers such as dioxane ortetrahydrofuran, or ketones such as acetone and methyl ethyl ketone.

Customarily, the concentrations of aqueous mineral acid and alkanol ordialkanol are selected such that on average not more than 1.01 mol,preferably from 1.00 to 1.005 mol of mineral acid equivalents per moleof hydroxyl group in the alkanol or dialkanol are used. In a veryparticularly preferred embodiment of the invention, an equimolarquantity of the mineral acid per mole of hydroxyl group in the alkanolor dialkanol is used.

The mixing of the alkanol and aqueous mineral acid can be carried out bythe usual means. The mixing temperature is relatively unimportant forthe success of the invention. In general, the temperature of the mixtureis set at that required for the subsequent reaction with the nitrite,i.e. at a temperature preferably in the range from 5 to 40° C., inparticular in the range from 10 to 30° C., especially in the range from15 to 25° C. If necessary, the mineral acid and the alkanol areprecooled to the desired temperature. The mixing of the alkanol ordialkanol and the mineral acid can take place in a container, in amixing chamber or via static mixers in a pipe. When a mixing containeris used for the preparation of the mixture, it can be carried out eithercontinuously or batchwise. The mixing can be carried out by stirring orby mixing of streams. In one embodiment of the invention, the alkanol ordialkanol are continuously fed into a container with mixing, preferablyinto a stirred-tank vessel, and continuously removed therefrom.

In the second step (ii) of the present process, an aqueous solution ofthe inorganic nitrite is continuously added to the aqueous mixtureobtained in (i) in a reaction zone.

Suitable sources of nitrite ions are inorganic nitrites, e.g. alkalimetal and alkaline earth metal nitrites such as sodium, potassium,barium and calcium nitrite, and also ammonium nitrite. Of these,potassium nitrite and especially sodium nitrite are particularlypreferred.

The quantity of inorganic nitrite added in the second step (ii)dissolved in water substantially corresponds to the requiredstoichiometry, although the nitrite can also be used in an excess thatin general does not exceed 20 mol %. The stoichiometry is based on theamount of hydroxyl group in the alkanol or dialkanol used. The molarratio of hydroxyl group in the alkanol or dialkanol to inorganic nitritegenerally is in the range from 1:1.0 to 1:1.2, preferably 1:1.01 to 1.2and in particular 1:1.05 to 1:1.12.

In general the total quantity of water in the second step (ii) is 1 to 5times, preferably 1.5 to 4 times and in particular 2.5 to 3.5 times thevolume of the organic phase.

To carry out step (ii), the mixture obtained in (i) and the aqueousnitrite solution are introduced into a reaction zone which has thedesired temperature. The reaction zone generally comprises one or more,successive reaction tanks (tank battery) or reaction tubes, and tanksand tank batteries are preferred. The reaction zones generally havestandard means for mixing liquids, e.g. static mixers such as internalsand/or stirrers and/or means for pumping liquids, preferably incombination with the static mixers. Mixing of the alcohol/acid mixtureand the nitrite solution can also be carried out by introducing theliquid reactants to the second step (ii) by means that are suitable forthe continuous mixing of liquids. In the case of relatively smallerapparatus dimensions, mixing of the reactants can also be achieved byintroducing the liquid reactants into the reactor with vortexing ofstreams of liquid, for example through two immediately adjacent inletsfor the streams of liquid. This method has proved itself particularly inlaboratory and “Miniplant” plants having reactor volumes (volumes of therespective reaction zone) of ≦1000 ml, preferably ≦200 ml.

Further, the reaction zone generally has standard means for removal ofthe heat of reaction, e.g. cooling coils, wall coolers and the like.

The residence time of the reactants in the reaction zone is in generalin the range from 20 minutes to 5 hours, preferably 25 min to 3 hours,in particular 40 min to 2 hours and very particularly preferably 40 minto 60 min. The residence time is by its nature dependent upon theaddition rate of the reactants. Normally, a longer residence time ischosen at low reaction temperatures and a shorter residence time ischosen at higher reaction temperatures.

The reaction temperature for the conversion of the alkanol or dialkanolinto the corresponding alkyl nitrite or dinitrite in the second step(ii) is preferably in the range from 5 to 40° C., in particular 10 to30° C. and especially 15 to 25° C.

The reaction mixture occurring at the downstream end of the reactionzone generally has a pH of above 3.

In a preferred embodiment of the invention, the reaction zone in step(ii) comprises at least two reaction tanks (battery). The mixtureobtained in step (i) and the aqueous inorganic nitrite solution are fedseparately and continuously into a first reactor. The contents of thefirst reactor are mixed by standard means, e.g. by stirring, intensivecirculation or by injection of the reactants. The mixture is transferredby means of a discharging device, e.g. an overflow, into at least onefurther reactor to complete the reaction, after a residence timedetermined by the relationship between the reactor volume and theintroduction and withdrawal rates. This method is particularlyadvantageous in the case of reactor volumes of at least 200 ml.

The product of value can be separated off directly after step (ii). Ingeneral, the reaction mixture is biphasic and readily separates. Theremoval of the product of value can be carried out by the standardtechniques, for example by introduction of the reaction effluent fromstep (ii) into a phase-separating vessel. The removal is normallycarried out at room temperature. The steps (i) to (iii) are normallycarried out at atmospheric pressure. The alkyl nitrite or dinitriteobtained generally has a GC (FID) purity of greater than 95%, veryfrequently greater than 97% and more often than not greater than 98%.The yield of the product of value is only slightly below the GC puritiesobtained.

The process of the invention has the advantage over the batchwiseprocesses of the prior art that the alkyl nitrite or dinitrite can beprepared with the use of a smaller excess of acid or without an excessof acid without loss of product quality and yield, so that thedisadvantages of the prior art outlined at the beginning are overcome.Furthermore, the continuous process allows a more efficient and rapidproduction of larger product quantities. Removal of the products ofvalue is not necessary, in contrast to the prior art batch processes, oronly in those cases where the use of water-free alkyl nitrite ondinitrite is required. The storage stability of the alkyl nitrite ordinitrite obtained is also better.

Furthermore, the process of the invention is more economical, since incontrast to the batchwise processes, little cooling if any is necessaryduring the reaction. Also, the aqueous phase does not need to beseparated from the product phase immediately after the reaction. Rather,it is even possible to store the product phase over the water phase atroom temperature for a limited period of several days, with onlynegligible decomposition occurring.

The process of the invention is illustrated by the following examples.

EXAMPLE 1

A 5 l glass feed vessel was charged with n-butanol and a second 5 lglass feed vessel was charged with 20% by weight of hydrochloric acid,and each were pumped by a membrane pump in a molar ratio of 1:1 into a0.75 l glass stirred reactor.

219.5 g/h of this mixture of n-butanol and hydrochloric acid and anaqueous 40% by weight sodium nitrite solution were pumped synchronouslyinto a first 0.75 l glass stirred reactor. The molar ratio of n-butanolto hydrochloric acid to sodium nitrite was 1:1:1.1. The mixture wastransferred through an overflow into a second 0.75 l glass stirredreactor after a residence time determined by the addition time. Thetotal residence time of the reactants was about 45 to 50 min. Thereaction mixture was finally transferred through an overflow into aseparating funnel. The n-butyl nitrite was separated off according to GC(FID) contained 0.77% of butanol. The purity of the product of valuewas, according to GC (FID), 98.87%. The quantity of dibutoxybutane wasbelow 0.1%. The product was stable for several days over the aqueousphase.

EXAMPLE 2

Example 1 was repeated, except that the butyl nitrite was not separatedfrom the aqueous phase until after 6 days. The storage over the aqueousphase was carried out at 5° C. The degree of decomposition of then-butyl nitrite was below 0.1%. Similar results were obtained afterstorage at 20° C.

EXAMPLE 3 COMPARATIVE EXAMPLE

Example 1 was repeated, except that the addition rate of the mixture ofHCl/BuOH was 324 g/h, and the molar ratio of butanol:HCl was 1:1.1. Thepurity of the product of value was 98.62% by GC (FID). The product ofvalue contained 1.01% of butanol.

EXAMPLE 4 COMPARATIVE EXAMPLE

Example 3 was repeated, except that the addition rate of the mixture ofHCl/BuOH was 233 g/h. Further, a 32% by weight solution of hydrochloricacid was used. The purity of the product of value was 98.72% by GC(FID). The product of value contained 0.93% of butanol.

EXAMPLE 5 COMPARATIVE EXAMPLE

Example 4 was repeated, except that the addition rate of the mixture ofHCl/BuOH was 232 g/h. Also, step (ii) was carried out at an internalreaction temperature of 10° C. The purity of the product of value was98.72% by GC (FID). The product of value contained 0.97% of butanol.

EXAMPLE 6 COMPARATIVE EXAMPLE

Example 5 was repeated, except that step (ii) was carried out at aninternal reaction temperature of 0° C. The purity of the product ofvalue was 99.02% by GC (FID). The product of value contained 0.93% ofbutanol.

EXAMPLE 7 COMPARATIVE EXAMPLE

Example 1 was repeated, except that the addition rate of the mixture ofHCl/BuOH was 203.5 g/h, and the molar ratio of butanol:HCl was 1:1.05.Also, step (ii) was carried out at an internal reaction temperature of−10° C. The purity of the product of value was 98.86% by GC (FID). Theproduct of value contained 0.88% of butanol. If the product was notseparated immediately from the aqueous phase, decomposition to n-butanoland the formation of dibutoxybutane (up to 3%) took place at 5° C.within about 2 days.

EXAMPLE 8 COMPARATIVE, BATCHWISE PROCESS

A tank was charged with 81.5 kg (1100 mol) of n-butanol and cooled to−5° C. 125.3 kg (1115 mol) of 32.4% hydrochloric acid were added over2.25 hours such that the temperature in the tank remained constantlybelow −5° C. The contents of the tank were then cooled at −15° C. and208.2 kg (1210 mol) of a 40% sodium nitrite solution that had beencooled to 8 to 12° C. were added with stirring over the course of about6 hours so that the temperature remained below −5° C. Once addition wascomplete, stirring was continued for a further 15 min at from −10 to −5°C., the stirrer was switched off in order to separate the phases and theorganic phase was separated off. The yield of n-butyl nitrite was 98.5%in a purity by GC (FID) of 98.25%. The product also contained 0.73%butanol by GC (FID). If the product was not immediately separated fromthe aqueous phase, decomposition to n-butanol and the formation ofdibutoxybutane (up to 3%) occurred at 5° C. within about 2 days.

1. A process for continuous preparation of alkyl nitrites and alkyldinitrites by reaction of an alkanol or a dialkanol with an inorganicnitrite in the presence of at least one mineral acid that does notoxidize nitrite, wherein (i) the alkanol or dialkanol is mixed with anaqueous solution of the mineral acid using on average not more than 1.01mol of acid equivalents per mole of hydroxyl group in the alkanol ordialkanol, (ii) an aqueous solution of the inorganic nitrite is addedcontinuously to the aqueous mixture obtained in (i) in a reaction zone,and (iii) the organic phase is optionally isolated.
 2. The process asclaimed in claim 1, wherein the aqueous mineral acid in step (i) ishydrochloric acid.
 3. The process as claimed in claim 2, wherein theconcentration of hydrogen chloride in water is in the range from 10 to40% by weight.
 4. The process as claimed in claim 1, wherein theinorganic nitrite in step (ii) is selected from the group consisting ofsodium nitrite and potassium nitrite.
 5. The process as claimed in claim1, wherein step (ii) is carried out at temperatures in the range from 5to 40° C.
 6. The process as claimed in claim 1, wherein the molar ratioof hydroxyl group in the alkanol or dialkanol to inorganic nitrite is inthe range from 1:1.0 to 1:1.2.
 7. The process as claimed in claim 1,wherein the residence time of the reactants in the reaction zone is inthe range from 20 minutes to 5 hours.
 8. The process as claimed in claim1, wherein the pH of the reaction mixture at the downstream end of step(ii) is above
 3. 9. The process as claimed in claim 1, wherein thealkanol or dialkanol is a linear or branched C₂C₆alkanol or dialkanol.10. The process as claimed in claim 9, wherein the C₂-C₆-alkanol isselected from the group consisting of n-butanol, sec-butanol,isobutanol, tert-butanol and isoamyl alcohol.
 11. The process as claimedin claim 9, wherein the C₂-C₆-dialkanol is neopentyl glycol.
 12. Theprocess as claimed in claim 1, wherein the acid equivalents per mole ofhydroxyl group in the alkanol or dialkanol are on average not more than1.00 mol.